1. Field of the Invention
The present invention relates to an information storage apparatus for storing/reproducing audio information such as speech or music information and, more particularly, to an information storage apparatus with speech coding and decoding functions.
2. Description of the Related Art
In a conventional information storage apparatus, although coding and decoding are performed before and after information storage, it is presumed that an original signal to be coded and a decoded/reproduced signal must have coding rules which coincide with each other. When the conventional information storage apparatus is connected to a digital channel in which pieces of information having different coding rules are present, the pieces of information are normalized and then stored because the coding rule of a read destination is not confirmed.
In particular, even when the coding rule of a storage source coincides with the coding rule of a read destination, when information is transmitted through an information storage apparatus for coding the information to normalize it and decoding the normalized code, audio quality obtained after reproduction is degraded compared with that obtained when communication is performed without going through an information storage apparatus.
For example, in a digital mobile communication switching unit for Japan and USA, a .mu.-1a.omega..multidot.PCM code is applied on a communication channel between the digital mobile communication switching unit and a fixed network. A low-rate code such as a VSELP code is used between the digital mobile communication switching unit and a mobile unit. In communication between the terminal of the fixed network and the mobile unit, .mu.-1a.omega.-VSELP code conversion is performed through a transcoder. In communication between mobile units, no conversion is performed, and so-called "codec through connection" is performed to avoid quality degradation caused by the conversion. However, when a storage apparatus is connected to such a switching unit, quality degradation is inevitable.
On the other hand, in order to effectively use a storage unit, the following system (e.g., Japanese Patent Laid-Open No. 63-296446) is provided. That is, a coding rule is switched to a desired compressing/coding rule in accordance with the request of a customer, and audio information is compressed and coded and then stored in the storage unit. During reproduction, information read from the storage unit is reproduced by a decoding unit using a coding rule corresponding to the compressing/coding rule.
However, when a conventional information storage apparatus is connected to a digital channel in which pieces of information having different coding rules are present, the coding rule of a storage source used when the information is stored in the storage apparatus may not correspond to the coding rule of a read destination for reproducing information read out from a storage unit. For this reason, code conversion must be performed, thereby degrading communication quality.